The present invention relates generally to a controller for AC motors. More particularly, the invention relates to a motor controller having a compact construction by virtue of a novel partition of functional circuitry between a power substrate board and a control board.
In recent years the popularity of AC motor controllers, particularly variable frequency motor controllers, has grown tremendously. Such controllers find application in virtually all facets of industry, including material handling, manufacturing and process control. Also referred to as motor drive packages, these controllers are designed for installation between a source of AC power and the motor to be controlled and offer the engineer considerable flexibility in adapting the drive characteristics of the controller to the particular application on which the motor is installed. Once practical only on applications involving large motors using complex hard-wired relay circuits, sophisticated motor controllers are now commonly available for small AC motors as well. Moreover, the use of solid state programmable circuitry has permitted even small motor controllers to offer even greater flexibility in a compact and economical package.
Known AC motor controllers of this type generally include a number of solid state power components, such as rectifying circuitry and inverting circuitry, typically including insulated gate bipolar transistors (IGBT's), as well as lower voltage isolation, drive and monitoring circuits for driving the IGBT's and for monitoring certain operating parameters of the drive. In addition, such controllers include a logic circuit board supporting components such as a microprocessor, and a power supply board supporting circuitry for supplying power to the switching devices and logic circuitry. While the lower voltage logic circuitry is thus typically separated from the higher voltage components, it is common in the art to support both the input rectifying circuitry, the inverting circuitry and the isolation, drive and monitoring circuitry on a single drive circuit board.
In operation, the controller is wired to input phase conductors and the rectifying circuit converts incoming AC power to DC power. The DC power is transmitted to the inverting circuit via a DC bus. The inverting circuit reconverts the DC power to AC power at controlled voltage, current and frequency in response to control signals generated by the logic control circuitry in accordance with preset control routines. The controlled AC power is then transmitted to the motor via output conductors to drive the motor.
While such motor controllers have proven extremely useful in many applications, further reductions in size and cost and improvements in efficiency are constantly sought, particularly for smaller horsepower controllers (e.g. 5 horsepower and below). However, circuit board configurations and internal architecture of current motor controllers inherently limit further significant size reduction. For example, placing control circuitry and power circuitry, including rectifying and inverting circuits, on a single, large hybrid circuit board, generally produces a large footprint for the resulting drive package.
Placing portions of the control circuitry on the same power substrate as the higher voltage rectifying and inverting circuits leads to other disadvantages as well. For example, because the latter circuits require significant cooling, they are commonly supported on relatively expensive coated aluminum substrate material mounted on a large heat sink spanning the entire power substrate in the assembled drive package. In addition, control circuitry does not generally have heat dissipation requirements as high as those of the rectifying and inverting circuits and could therefore be supported suitably on less expensive epoxy-fiberglass laminated substrate. Thus, the inclusion of control circuitry on the power substrate results in inefficient use of substrate material as well as heat sink material.
Moreover, further size reduction in conventional motor controller circuit boards is limited by standards respecting clearance and creepage. Such standards generally impose minimum distance requirements for spacing between circuit components and across the circuit board surface, respectively, based on environment classes and voltage ratings for the circuits. Thus, by placing control circuit elements on the power substrate with higher voltage rectifier and inverter circuits, wider spacing requirements are imposed on the control circuit elements than would be otherwise dictated by the reduced voltage of those elements if separated from the power substrate, further increasing the footprint of the controller circuitry.